1. Field of the Invention
This invention relates to a process for producing a propylene polymer or copolymer. More specifically, the invention relates to a commercially advantageous process for producing a propylene polymer or copolymer by polymerizing or copolymerizing propylene in liquid propylene (to be referred to hereinafter sometimes as "bulk polymerization").
2. Description of the Prior Art
Bulk polymerization methods are well known and have come into widespread commercial use. On comparison with a polymer slurry which is obtained in a conventional suspension (or solution) polymerization method (to be referred to hereinafter as the conventional solution polymerization method) using a liquid saturated hydrocarbon containing at least 5 carbon atoms, preferably 5 to 7 carbon atoms, as a solvent, the polymer slurry obtained in bulk polymerization has the advantage that the polymer can be very easily separated from the unsaturated hydrocarbon monomer (mainly propylene) solvent by simply subjecting the polymer slurry to a reduced pressure, because the unsaturated hydrocarbon monomer has a higher vapor pressure and is readily volatilizable.
However, the polymer obtained in such a bulk polymerization method generally has the disadvantage that it contains large quantities of a polymer soluble in boiling n-heptane and the catalyst residue. Generally, bulk polymerization has a higher rate of polymerization than conventional solution polymerization, and can be operated under conditions at which the catalyst efficiency is good. Thus, intrinsically, reduced residual catalyst results in the bulk polymerization method.
Accordingly, if under such conditions, a step of post-treating the polymerization reaction mixture for removal of the boiling n-heptane-soluble polymer could be eliminated, its economic effect would be very great. Polymers obtained by such a method have one or more of the defects mentioned below, and have inferior quality and economic value from a commercial standpoint.
For example, when a catalyst system composed of titanium trichloride and triethylaluminum is used, the polymer obtained by the above method contains only 75 to 85% by weight of a polymer insoluble in boiling n-heptane. With a catalyst system of titanium trichloride and diethylaluminum chloride, the amount of the insoluble polymer is 85 to 95%. Since such polymers have poor rigidity and antiblocking property, they cannot be used in applications which require relatively good characteristics, for example, in forming films or fibers, although they can be used for general molding purposes. In order to improve the characteristics of the polymer, the general practice is to contact the polymer slurry withdrawn from the polymerization system, or the polymer separated from the slurry by separation of the unsaturated hydrocarbon monomer (mainly propylene) at reduced pressure, with a suitable catalyst-decomposing agent and to wash the polymer with a solvent consisting mainly of a saturated hydrocarbon containing at least 5 carbon atoms, preferably 5 to 7 carbon atoms, or a liquid unsaturated hydrocarbon monomer (mainly liquid propylene) which is the same as the polymerization system, thereby extracting and removing the solvent-soluble polymer and a part of the residual catalyst.
Suitable catalyst-decomposing agents serve to stop the polymerization reaction by decomposing the active catalyst or forming a complex with the catalyst, thereby inhibiting the formation of a high molecular weight polymer which markedly degrades the properties of the final polymer, and also rendering the catalyst readily extractable by a solvent in the subsequent washing step. Generally, catalyst-decomposing agents include various alcohols, ethers, ketones and chelating agents which may, if desired, be diluted with suitable diluents (generally, the same solvent used in the above steps is employed as a diluent).
Methods have also been suggested to reduce the amount of polymer which is soluble in the polymer slurry in the polymerization system. They include, for example, a method which involves using a stereospecific catalyst system such as a complex of a titanium halide and an aluminum halide, a method which involves adding various compounds to be described below as a third component to the above catalyst system or polymerization system, and a method which involves suitably changing the polymerization conditions, for example, decreasing the polymerization temperature. According to these methods, however, the amount of the polymer soluble in the polymer slurry is still fairly large, and it is difficult to omit the step of removing the soluble polymer.
Previously it was noted that when a polymer slurry obtained by bulk polymerization was treated with liquid propylene as a washing solvent in a countercurrent-type washing tower, the polymer dissolved in the slurry, that portion of the residual catalyst which was dissolved in the polymer slurry, and a part of the residual catalyst which is precipitated in the polymer slurry can be easily separated. This led to the discovery of commercially advantageous methods for producing a propylene polymer or copolymer (e.g., as disclosed in Japanese Patent Application (OPI) Nos. 79589/75, 139986/76 and 3679/77, and U.S. Pat. No. 4,053,697).
These methods, however, tend to give rise to a problem when a very high level of purity beyond that generally in current commercial demand is required.
For example, film-grade polypropylene, which accounts for a great proportion of the uses of polypropylene, having an increased content of a polymer with a low degree of stereo-regularity, suffers from the defect that a film prepared therefrom sticks to itself ("blocking phenomenon"), and becomes useless.
There is a good correlation between the blocking phenomenon and the content of the cold xylene-soluble portion (to be abbreviated hereinafter as CXS (%) and determined as hereinafter described) of the resulting polymer. FIG. 1 of the accompanying drawings shows the relationship between CXS (%) and blocking in a propylene homopolymer and a random copolymer of propylene and ethylene (ethylene content: 2.5 to 4% by weight). The solid line shows the relationship for the homopolymer, and the broken line shows the relationship for the random copolymer. In order to reduce blocking to a practically permissible degree (30 g/100 cm.sup.2), it is necessary to adjust the CXS (%) to about 3 to 4% or less for the homopolymer, and 4 to 5% or less for the random copolymer. The CXS (%) has been chosen as a measure of the content of a polymer with a low degree of stereoregularity, because it has been found that a definite relationship does not always exist between the content of the boiling n-heptane-soluble portion which has been heretofore widely used, and blocking. For example, some polymers have a small CXS (%) and reduced blocking even when their content of the boiling n-heptane-soluble portion is large, and others show quite a contrary relationship.